U.S. Pat. No. 3,842,194 of Jon K. Clemens disclosed a video disc having a playback system utilizing variable capacitance. In one configuration of the Clemens system information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. For example, groove widths about 3.5 micrometers and groove depths of about 1.0 micrometer may be used. During playback a pickup stylus about 2.0 micrometers wide having a thin conductive electrode thereon, for example, about 0.2 micrometers thick, engages the groove as the record is rotated by a supportive turntable. Capacitive variations between the stylus electrode and the record surface are sensed to recover the pre-recorded information.
In the system of the above type the use of a relatively fine record groove and the groove engaging requirements for the pickup stylus results in a stylus tip which is extremely small.
In a copending application of Keizer entitled, "Keel-Tipped Stylus for Video Disc Systems", Ser. No. 781,317, filed Mar. 25, 1977, now U.S. Pat. No. 4,162,510 and incorporated herein by reference, a novel keel-tipped pickup stylus structure is disclosed. The keel-tipped pickup stylus comprises a dielectric support element having a body, a fore and aft constricted terminal portion and shoulders interconnecting the body with the constricted terminal portion.
In a second copending application of Keizer entitled, "Method for Forming Keel-Tipped Stylus for Video Disc Systems", Ser. No. 781,302, also filed Mar. 25, 1977, now U.S. Pat. No. 4,104,832, issued Aug. 8, 1978 and incorporated herein by reference, a flattened pyramidal shaped support element is shaped on an abrasive lapping disc having a deep, coarse-pitched groove for a predetermined time interval in order to obtain a keel-tipped stylus. Glow discharge deposited SiO.sub.2 was used by Keizer as the abrasive coating and diamond was used as the dielectric stylus support material. In the second Keizer application, the SiO.sub.2 layer is prepared by a method which utilizes as starting material oxygen and a dielectric precursor of an alkoxy-substituted silane of the formula ##STR1## wherein R.sub.1 is selected from the group consisting of H and CH.sub.3, R.sub.2 and R.sub.3 are independently selected from the group consisting of H, CH.sub.3, OCH.sub.3 and OC.sub.2 H.sub.5, and R.sub.4 is selected from the group consisting of OCH.sub.3 and OC.sub.2 H.sub.5.
These coatings are not sufficiently abrasive and as a result periods as long as 30 minutes are needed to shape one diamond stylus. Furthermore, these coatings quickly lose their abrading ability and a second diamond stylus may require 2 hours to be lapped. Therefore, a more abrasive, long lasting, conformal coating fine enough to lap 2.0 micrometer wide diamond styli would be desirable.